With Biden still in the process of getting to yes, the subtext of tonight’s debate is likely to be: does he have an opening or doesn’t he? And the answer to that question really has two parts: how strong does Clinton look, and how weak does Obama look?

If the dynamic on stage is a commanding, experienced Hillary Clinton facing a passionate, populist insurgent and a handful of irrelevant, hopping fleas, then Clinton looks strong, and Biden has less of an opening. Similarly, if all of the candidates on stage take pains to run against the Obama legacy – from whatever direction – then they have clearly made the calculation that his legacy is a net-negative, and, again, Biden has less of an opening.

But, if the dynamic on stage is a beleaguered Hillary Clinton facing challenges from all sides on her hawkish foreign policy and her closeness with Wall Street, and she responds testily or incoherently to these challenges, then there’s more clearly an opening for Biden to get in. And if the Democratic candidates are more united in blaming the Republicans for the unsatisfying aspects of the Obama years, and stress the vital importance of denying them the White House, then that suggests an awareness of the need to tread carefully in criticizing Obama in a Democratic primary, and, again, Biden has more of an opening.

So which will it be?

I think we have a pretty good idea of how Sanders is going to operate on the stage. He’s going to make the case for a sharp left-turn without personally being especially critical of either Obama or Clinton. And I think I can guess how Clinton would respond if Sanders were the only one on stage with her – she’d avoid engaging in ideological argument, but make the pragmatic case for herself as someone who can advance progressive ideals most effectively. And if that’s the game, then there’s little opening for Biden.

All of which means that tonight will be the first, and possibly last, chance for the until-now hopeless candidates to shape the race. If Webb and O’Malley are out for blood, they will draw it. The openings are there, in both foreign and domestic policy. And then we’ll see how Clinton – and Sanders – respond once there’s blood in the water.

Meanwhile, what I am really hoping for is that Chafee gets a question about the metric system.